Virtually all valves for controlling the flow of fluids in piping systems comprise a valve body structure having a valve element therein that is capable of being moved between open and closed positions. In most cases, the valve is provided with a valve stem that extends through the valve body and is connected to the internal valve element. In order to prevent leakage between the valve stem and valve body, the valve mechanism is typically provided with a stem packing assembly that is received within a packing chamber or stuffing box and which encircles the valve stem. Packing assemblies may develop sealing characteristics responsive to application of mechanical pressure thereto or, in the alternative, application of fluid pressure thereto may enhance the sealing capability of such packing assemblies.
Most packing assemblies for valves incorporate sealing materials such as rubber or rubber-like material, plastic, and in many cases lubricant materials. The function of the packing assembly is to completely fill the space between the valve actuator stem and valve body structure to thus prevent liquid or gaseous materials from escaping through the valve stem opening of the valve. The packing assembly must also allow movement of the valve stem, either linearly in the case of gate valves, rotatably in the case of plug valves, and in some cases both linearly and rotatably in the case of lift turn plug valves. Most packing assemblies that are available at the present time, have the capability of achieving adequate sealing capability while at the same time allowing the valve stem freedom of movement for the purpose of valve actuation.
In the petroleum and petrochemical industries, flow lines often transport flammable material that will feed a fire that has become ignited. Where valves are employed to control the flow of such hazardous fluids, it is highly desirable that at least some of the valves have the capability of maintaining effective sealing even under circumstances where the piping system and valve is subjected to extreme external heat such as typically occurs when a fire has become ignited. In most cases, the packing assemblies of valves are incapable of withstanding extreme heat and the sealing materials deteriorate rapidly, thereby allowing leakage past the valve stem packing. Under circumstances where the flow system is maintained under significantly high pressure, the flammable or otherwise hazardous liquid will leak past the packing at a significantly high rate to feed the fire. This is detrimental to fire fighting and to the safety of personnel in the immediate vicinity. If some of the valves of the flow system are provided with packing assemblies that are effectively resistant to extremely high temperatures, these valves may be positioned in the open or closed positions thereof as desired for purposes of efficiency in fighting the fire and for the purposes of safety to personnel. Since the valve stem packing will maintain its sealing capability even when heated to an extremely high temperature, the flammable fluid of the flow line will be effectively controlled at least for a sufficient period of time to allow the fire to be brought under control.